A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus movable objects, such as a movable substrate support and a movable patterning device support are used. These movable objects may be moved with high accuracy. To determine the position of the movable objects, position measurement systems capable of measuring a position with high accuracy are provided. For example, interferometer systems and encoder measurement systems have been provided for high accuracy position measurement of movable objects in a lithographic apparatus.
An encoder-type measurement system comprises at least one encoder head and a grating, for example arranged on a reference plate or other reference element. The encoder head may be any type of encoder sensor capable of reading the grating.
The encoder head may be mounted on a first object and the grating on the second object. The grating comprises periodic array of grating lines, and the encoder head is configured to read the periodic array of grating lines in order to determine a change in a relative position of the first object with respect to the second object by counting the grating lines that pass during a relative movement. In some embodiments, the encoder head may determine, within a range of a relatively small number of grating lines, an absolute position with respect to this range of grating lines. Since, globally, the encoder head can only determine a position change of the encoder head with respect to the grating and not an absolute position, it is desirable to know a zero position or start position of the encoder head with respect to the grating in order to make determination of an absolute global position of the first object with respect to the second object possible.
The grating may be a two-dimensional grating configured to be used in at least two measurement directions. Such two-dimensional grating comprises a first array of grating lines in a first direction and a second array of grating lines in a second direction, wherein the first array of grating lines and the second array of grating lines overlap. The first direction and the second direction are perpendicular with respect to each other. The first array of grating lines and the second array of grating lines diffract a measurement beam incident on the grating in at least one first diffracted beam in the first direction and at least one second diffracted beam in the second direction. The at least one first diffracted beam and/or at least one second diffracted beam are received by the encoder head and used for position measurement in the first direction and the second direction, respectively.
The grating may be used by different encoder sensors having different measurement directions in the plane of the grating.
In the known encoder-type position measurement system having a two-dimensional grating, with increasing number of encoder heads which may be desirable to increase overall accuracy of the position measurement system, more laser power is desired to use the encoder heads adequately. This laser power is provided by one or more laser sources, for example laser boxes. Increasing laser power requires a laser source having increased power or multiple laser sources. A laser source having increased power or multiple laser sources may substantially increase costs, weight and volume for integration of the laser source or sources. Multiple laser sources further necessitate synchronization of the different laser sources.
Further, in the known encoder-type position measurement system, the known grating is not suitable to determine an absolute global position of the encoder head with respect to the grating without having a negative influence on high accuracy measurement at the location where an absolute global position is measured.
In an embodiment of a known position measurement system a separate absolute position sensor is provided which is configured to determine an absolute position of the encoder head with respect to the grating. This absolute position sensor comprises a sensor which is configured to read a mark arranged at a fixed position with respect to the grating.
In another known embodiment, the grating itself is provided with a local mark. When the encoder head of the position measurement system is aligned with this local mark, the encoder head will determine the presence of this mark, and, as a result, an absolute position of the encoder head with respect to the grating can be determined. However, the presence of such local mark has a negative influence on high accuracy measurement at the area of the local mark. Therefore, such local mark is not suitable for determination of an absolute position in areas where high accuracy measurement is desirable.